This invention relates to the simultaneous scrubbing and oxidation of hydrogen sulfide, and in particular to the prevention of the formation of thiosulfate during such a process. In such processes, an aqueous alkaline scrubbing agent containing an oxidizing agent is employed, and the hydrogen sulfide is absorbed from the scrubbing agent in a scrubbing column and oxidized principally to elementary sulfur with reduction of the oxidizing agent. The scrubbing agent generally is subsequently regenerated for reuse by treatment with an oxygen-containing gas for reoxidation of the oxidizing agent.
In known processes of this type, the gas to be cleaned of hydrogen sulfide is scrubbed with an aqueous alkaline solution, such as a sodium carbonate solution having a pH between 8 and 9, which contains a soluble salt, such as the sodium salt of anthraquinonedisulfonic acid, and a metal vanadate, the vanadium being present in the five-valent oxidation state (for example British Patent Specification No. 1,488,659). The absorbed hydrogen sulfide dissociates in the solution with formation of HS.sup.- ions which reduce the vanadium to the four-valent oxidation state, elementary sulfur being liberated. The reduced vanadium can be reoxidized by an oxidation reaction with an oxygen-containing gas in the presence of anthraquinonedisulfonic acid.
It has been found, however, that part of the hydrogen sulfide is oxidized to soluble sodium sulfate and sodium thiosulfate which build up in the scrubbing solution over time and must be removed. Whereas the removal of sodium sulfate can be accomplished, for example, by simple crystallization, this is not possible for sodium thiosulfate because of its high solubility. Furthermore, since sodium thiosulfate is a substance which is ecologically deleterious because of its high biological oxygen demand, the scrubbing solution cannot simply be discarded.
In this context, it has already been proposed (German Patent Specification No. P 32 16 160.3), to shift the ratio of formation of sulfate to thiosulfate ions in favor of the sulfate ions by decreasing the pH in the scrubbing solution, and thus to suppress the formation of thiosulfate.